Inflated concrete shell enclosures have been built by various methods, including:
1. Inflating a bag, covering it with reinforcing mesh, spraying it with concrete, then removing the bag through an opening in the hardened shell for reuse.
2. Inflating a bag containing an air lock door, spraying the inside of the bag with insulating foam, spraying the inside of the insulating foam with concrete, then removing the bag from the outside for reuse, or leaving the bag as a permanent water proof covering (U.S. Pat. No. 3,277,219).
3. Placing concrete and an expandable reinforcing members on an elastic membrane at ground level, then inflating the membrane and vibrating the surface to insure continuity of concrete over the expanded surface area of the resulting shell (U.S. Pat. No. 3,462,521).
4. A method combining elements of methods 1 and 3, in which reinforcing and a thin layer of concrete are placed on the elastic membrane at ground level and the membrane is inflated. When the thin layer has hardened, and while the membrane is still inflated, a second layer of reinforcing and a thicker layer of concrete are applied over the thin layer (U.S. Pat. No. 4,170,093). The advantages claimed for this method are that it allows larger shells to be formed than does method 3, since the thin layer of concrete can be placed on a larger surface area and inflated before it begins to harden whereas a thicker layer could not, and that it avoids the scaffolding required for the guniting or shotcreting used in method 1, since the combination of inflationpressure and hardened thin layer provide a surface rigid enough for workmen to walk upon.
5. U.S. Pat. No. 3,643,910 describes perimeter framing techniques for inflated shell construction, including one scheme consisting of an inflatable double membrane between upper and lower tensioning cables extending across and anchored to opposite sides of a circular perimeter frame. The cables contain and determine the shape of the inflated double membrane. Concrete is poured on the upper membrane, either before or after inflation, and the upper cables become embedded in the concrete to serve as shell reinforcement.
The perimeter ring may be supported on a pre-constructed ring wall, before inflating the membrane and placing concrete. However, since the upper and lower membrane are constrained between the upper and lower tensioning cables, and since the tensioning cables cannot be removed because they are integral and necessary structural elements of this system, neither upper nor lower membrane can be made to react against a supporting base in order to raise the hardened shell in a manner to be described in the present invention.
Some disadvantages of all of the cited methods include:
1. The required use of a wet concrete mix and the attendant time limitations imposed by hardening of the concrete. In methods 1 and 2 either all concrete must be placed before any of it begins to harden, since the bag deforms under the weight of concrete, or else a very rigidly inflated bag, requiring expensive membrane and anchorages must be used. In methods 3 and 4, the concrete must be placed, the bag safely inflated, and in method 3 the concrete adequately vibrated, before the concrete begins to harden. A construction delay for any reason (delayed concrete delivery, faulty air seal, power outage, unfavorable weather) between placing the first concrete and having all concrete in its final position for hardening may void or reduce the structural integrity of the shell.
2. The large investment of labor required to place and properly anchor numerous individual reinforcing members and the cost in materials and time to properly position the reinforcing members within the thickness of the shell cross section. The reference for method 4 additionally describes the need to bond the two concrete layers together with an adhesive and to tie the two reinforcing layers together.
3. The required cost of specialized equipment and skilled labor for guniting or shotcreting for all of the cited methods except method 3, in which method the shell size is limited by the concrete hardening duration constraint previously described.
4. For some applications, the restricted usefulness of the shell enclosure due to limited headroom near the perimeter and the increased cost of acceptable window and entry installation due to the lack of vertical walls. Method 5 overcomes this limitation, but at the cost of having to place concrete above ground level, requiring pumped concrete or alternative means of lifting wet concrete.